Add a section to the readme on linting

- Update contributing.md
- Add section on linting, include details of GOV.UK lint and Standard
JS.

CONTRIBUTING.md

@@ -5,6 +5,7 @@ guidelines:
 
 ## Indentation and whitespace
 
+Your JavaScript code should pass [linting](https://github.com/alphagov/govuk_prototype_kit/blob/0d9835b7ad8d7bc06aa288204eb3ce9a07116be2/docs/linting.md).
 2-space, soft-tabs only. No trailing whitespace.
 
 ## Versioning

README.md

@@ -150,6 +150,22 @@ Run the app
 Go to [localhost:3000](http://localhost:3000) in your browser.
 
 
+## Linting
+
+### GOV.UK lint
+[GOV.UK elements uses govuk-lint](https://github.com/alphagov/govuk-lint#sass), which uses [scss-lint](https://github.com/brigade/scss-lint) as its scss linter.
+
+### Standard JavaScript
+GOV.UK elements uses [standardjs](http://standardjs.com/), an opinionated JavaScript linter. All JavaScript files follow its conventions.
+
+Both linters run on CI to ensure that new pull requests are in line with them.
+
+## Linting
+
+To check the whole codebase, run:
+
+    npm test
+
 ## Running Wraith to compare changes
 
 GOV.UK elements uses Wraith so that regressions can be easily spotted.